Water Impact Loads And Dynamic Response Of Horizontal Circular Cylinders In Offshore Structures

Abstract

ABSTRACT Model test results and numerical calculations of water impact loads on rigid horizontal circular cylinders are presented. The initial slamming force coefficient Cs is numerically estimated to be 3.1. Experiments give an average value of 5.3. The time history of the water induced load is examined. Dynamic response of three-dimensional elastic models of horizontal trusses of semisubmersibles and jackets are studied. Scaling of model test results result in significant stresses for realistic wave velocities in the North Sea. INTRODUCTION The wave-induced loads are an important factor in the design of marine structures for offshore operation in the hostile area of the North Sea. For some types of structures and wave induced loads the' loads can be well predicted. For other cases this is not true. In particular the calculation of wave loads on jackets is a difficult task. This paper will have particular application to the members of a jacket that are in the free surface zone. Ore to the wave notion those members will be in and out of the water. Cases of damages on this type of members have been reported from the North Sea. Similar problems can arise for the horizontal trusses of a semisubmersible. In order to be able to deal with the problem we will consider a simplified problem. We will study a horizontal circular cylinder that is forced with constant velocity through an initially calm water surface and examine the force on the cylinder as it proceeds through the water. One of the first calculations of this type was done by van Kaman (1), who considered a wedgeshaped body. He used the free surface condition ? = 0, where ? is the velocity potential of the fluid notion. The force was written as(Mathematical equation available in full paper) where m' is vertical infinite frequency added mass coefficient for the body as a function of time. Von Kaman simplified the added mass calculations by using flat plate results. Fabula and Ruggles (2) did similar calculations for a cylinder with elliptical cross-section, but their added mass calculations were more sophisticated than von Karman' s calculations. Fabula' s method is only applicable up to the point that a circular cylinder bottom has proceeded a distance approximately a radius down in the fluid. In this paper we will present as method which is applicable for a rigid cylinder proceeding any distance down in the fluid. The method may be used for any cross-section and would hence be applicable in the study of whipping of ships. The theoretical calculations are compared with experimental results obtained by forcing rigid horizontal circular cylinders with constant velocity through an initially calm free surface. In the second part we study the dynamic response of elastic horizontal circular cylinders that is forced with constant velocity through the water surface. A theoretical model is described. In the experimental part we are studying three-dimensional models of horizontal trusses of semisubmersibles and jackets.